adapt to changes in magit/magit

Hi Yann

I have recently made some changes to Magit that require changes to this extension and others. I will also likely make some other such changes soon.

Should I create pull requests for all these or should we make `magit/*` instead of `sigma/*` the repository used to build Melpa packages? I would prefer the latter since you are not actively working on these extensions right now and being able to push to "the main repository" would make it easier for me. (I won't make any big changes, just those required for compatibility with changes in Magit.

Cheers,
Jonas

Ps: `magit-tramp` is a really cool idea.